Practical_Electronics-May_2019

where BFR is the basic failure rate (% per

1,000 hours), VW is the actual working

(operating) voltage (in volts), and VMR is

the manufacturer’s rated voltage.

Hence, the effective failure rate for

capacitor A will be:

While the effective failure at for capacitor

B will be:

From this it is worth noting that, even

with a lower basic failure rate, capacitor

B is over three times more likely to fail

than capacitor A!

Dissipation factor and ESR

Table 6.2 lists typical electrolytic

capacitor characteristics in relation to

voltage rating. Note how dissipation

factor decreases as the voltage rating

increases. Higher-voltage electrolytics

tend to have a larger case sizes together

with lower ESR values (see Table 6.2).

As a result, internal temperature and

dissipation factor can be signifi cantly

reduced by selecting a component with

a higher voltage rating than is strictly

necessary for a given application. By

limiting internal dissipation, the working

temperature of the capacitor is reduced,

enhancing dielectric life and ensuring

long term reliability. Thus, when the time

comes to select an electrolytic component

for use as a reservoir capacitor or ripple

fi lter, it is generally wise to apply some

voltage de-rating.

recommended maximum value. With

older electrolytic capacitors it was

possible for the dielectric to slowly

deteriorate when the capacitor was

operated at signifi cantly less than its

rated value. Nowadays, use of high purity

aluminium foil has largely eliminated

this effect. Indeed, the effect of using

an electrolytic capacitor at less than its

rated value is to increase the reliability of

the dielectric in accordance with a fi fth

power law. It’s worth illustrating this

with two contrasting examples:

Capacitor A

Capacitor value ......................10,000μF

Manufacturer’s voltage rating ......100V

Actual working (operating) voltage .. 67V

Basic failure rate ............0.1% / 1,000 hrs

Capacitor B

Capacitor value ........................6,800μF

Manufacturer’s voltage rating ........50V

Actual working (operating) voltage .. 50V

Basic failure rate ......0.05% per 1,000 hrs

The effective failure rate (EFR) for a

capacitor can be calculated from:

Fig.6.11. Some low-cost component

checkers incorporate capacitor ESR

measurement. Here a good quality

4700μF axial lead capacitor is

being tested.

×

−

××−

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

×

×

× × ×

5

W

MR

EFR BFR V
V

= ×⎛⎞

⎜⎟⎝⎠ % per 1,000 hours

× ×⎛⎞

⎜⎟⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

×

−

××−

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

×

×

× × ×

×⎛⎞

⎜⎟

⎝⎠

5

67

0.1 0.1 0.135

100

EFR

⎛⎞

= ×⎜⎟= × =

⎝⎠

× ×⎛⎞

⎜⎟⎝⎠

×

−

××−

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

×

×

× × ×

×⎛⎞

⎜⎟⎝⎠

× ×⎛⎞⎜⎟ 5 0.0135% per 1000 hours=

⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

×

−

××−

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

×

×

× × ×

×⎛⎞

⎜⎟

⎝⎠

× ×⎛⎞

⎜⎟⎝⎠

1005

0.05 0.05 1

100

EFR= ×⎛⎞⎜⎟= × =

⎝⎠

×

−

××−

−

−

×× × −

−

π

π

⎛⎞

⎜⎟

⎝⎠

π ××× ×−

×

×

× × ×

×⎛⎞

⎜⎟

⎝⎠

× ×⎛⎞

⎜⎟

⎝⎠

× ×⎛⎞⎜⎟ 1 0.05% per 1000 hours=

⎝⎠

Voltage rating (V) 10 16 25 35 50 63

Dissipation factor (D) 0.19 0.16 0.14 0.12 0.1 0.1

ESR (Ω) 1.15 0.97 0.84 0.72 0.6 0.6

Max ripple current (mA) 260 320 430 480 540 580

Voltage rating (V) 10 16 25 35 50 63

Dissipation factor (D) 0.15 0.1 0.08 0.07 0.06 0.05

ESR (Ω) 0.5 0.4 0.32 0.28 0.22 0.21

Max. ripple current (mA) 380 450 600 700 900 1,100

Table 6.2 Typical characteristics for conventional

electrolytic capacitors

Table 6.3 Typical characteristics for ‘low-ESR’

electrolytic capacitors

Practi cal Proj ect: 9V-to-15V

CMOS Logi c Suppl y Conver ter_____

This month’s Practical Project takes the

form of a 9V-to-15V voltage converter

based on a low-cost 555 timer and

voltage doubler. This circuit is ideal for

supplying power to CMOS logic devices

that require a nominal 15V supply. The

specifi cations are shown in Table 6.4.

Table 6.4 Specifi cations for the 9V-to-

15V CMOS Logic Supply Converter

Input voltage ...............................................9V

Output voltage .............................15V (±0.5V)

Load current ........15mA nominal (25mA max)

Switching frequency ...................32kHz approx

Load regulation .............................13% approx

Output resistance ........................ 100 Ω approx

The circuit of our 9V-to-15V CMOS Logic

Supply Converter is shown in Fig.6.12.

A 555 timer (IC1) is used as an astable

oscillator with its operating frequency

determined by R1, R2 and C2. The square

wave output from pin-3 at approximately

32kHz is fed to a simple voltage-doubler

arrangement comprising D2/D3 and C4/

C5. The fi nal DC output appearing across

C5 is approximately double that of the

supply (less the forward voltage drops

associated with D2 and D3).

You will need...

Perforated copper stripboard (9 strips

each with 25 holes)

2 2-way PCB screw terminal connector

(ST1 and ST2)

1 1kΩ resistor (R1)

1 22kΩ resistor (R2)

3 100μF 35V capacitors (C1, C4 and C5)

1 1nF ceramic capacitor (C2)

1 100nF ceramic capacitor (C3)

1 555 8-pin DIL timer (IC1)

1 low-profi le 8-pin DIL socket

3 1N4148 diodes (D1 to D3)

1 15V Zener diode (optional, see text)

4 stand-off pillars and mounting screws

Construction

The stripboard layout of the 9V-to-15V

CMOS logic supply converter is shown

in Fig.6.13. Note that there are 23 track

Fig.6.12. Teach-In Practical Project: 9V-to-15V CMOS Logic Supply Converter.

(To improve output voltage accuracy, a 15V Zener diode (in red) can be added.)